Compound Interest: The Chemistry of Sparklers

It’s Fireworks Night here in the UK tomorrow, which means fireworks (obviously), bonfires and sparklers. We’ve looked at fireworks in a previous post, so this time around it’s time to take a look at the chemicals that go into producing sparklers, and their various roles.

In sparklers, there are three main components to the composition: a metal fuel, an oxidiser, and a binder. There are also other optional components that can be included in the mixture in order to modify the effect. The wire that forms the body of the sparkler is often made of iron, and the afore-mentioned composition is introduced to this wire as a paste, in which the wire can be coated.

The oxidiser in the sparkler plays an important role; they are often metal nitrates, chlorates, or perchlorates, with potassium nitrate being a very common example. When heated, these oxidiser compounds decompose, and when they do so they all, regardless of their identity, produce oxygen as one of the products of the decomposition. The production of gases during the decomposition reaction also forcibly ejects bits of the burning powdered metal from the sparkler. This causes the sparkler’s effect, as they continue to burn, and react with oxygen to produce metal oxides.

As well as being important for the production of sparks, the powdered metals used in the metal fuel can also affect the colour of the sparkler. Aluminium, magnesium and titanium all give near brilliant white sparks; iron, on the other hand, produces orange sparks, whilst ferrotitanium (an iron and titanium alloy) produces yellow-gold sparks. If this isn’t enough of a range of colours, optional chemical colourants can also be added. These are similar to the colourants used in large fireworks, salts of various metals that can impart greens, blues and reds. We summarised some of the different compounds that can be used for this purpose in the older post on the colours of fireworks.

Of course, the metal powder and the oxidiser would be useless without a way to bind them together into a paste that can then be added onto the metal rod that forms our sparkler. For that, a binder is needed to physically hold the chemicals together. A number of chemicals can be used for this purpose, but the most common is dextrin. Dextrin is actually a name for a group of smaller carbohydrates produced by the breakdown of starch; it can also act as a fuel, though in the amounts used in sparklers it doesn’t really contribute a great deal to the pyrotechnics. Water is also required in order to form a paste, though obviously the sparklers must consequently be dried before they can be burned.

Gunpowder is a key player in bigger fireworks, as we’ve noted previously, but it doesn’t really play a role in sparklers. However, some of its component parts, namely charcoal and sulfur, can crop up as additional fuels. They act as reducing agents, and burn the oxygen produced by the oxidisers; this can help modify the burning time of the sparkler. The temperature of the burning sparkler can easily reach between 1000-1600˚C, and the use of additional fuels can also have an effect on this.

Regardless of your previous chemistry education, you probably came across the concept of oxidation and reduction reactions. Well, sparklers essentially boil down to utilising a combination of oxidation and reduction reactions in order to produce their effect. So, whether you’re planning on heading to a big fireworks event for Fireworks Night, or just having a small firework display of your own in the back garden, you’ll now be able to do so with a renewed appreciation of sparklers and the chemistry behind them!